In a cellular communications network, user communication devices (also known as User Equipment (UE) or mobile terminals, such as mobile telephones) communicate with remote servers or with other user communication devices via base stations. An LTE base station is also known as an ‘enhanced NodeB’ (eNB). When a user communication device attaches to the LTE network via a base station, a core network entity called Mobility Management Entity (MME) sets up a default Evolved Packet System (EPS) Bearer between the user communication device and a gateway in the core network. An EPS Bearer defines a transmission path through the network and assigns an IP address to the user communication device to be used by the user communication device to communicate with remote servers or other user communication devices. An EPS Bearer also has a set of data transmission characteristics, such as quality of service, data rate and flow control parameters, which are defined by the subscription associated with the user communication device and are established by the MME upon registration of the user communication device with the network.
The EPS Bearer is thus managed by the MME, which signals to the user communication device when it needs to activate, modify, or deactivate a particular EPS Bearer. Thus there are two connections between the user communication device and the communication network: one for the user data transmitted using the established EPS bearer (also known as the user plane or U-plane) and another one for managing the EPS Bearer itself (also known as the control plane or C-plane)
In order to optimise utilisation of their bandwidth, LTE base stations receive periodic signal measurement reports from each served user communication device, which contain information about the perceived signal quality on a given frequency band used by (or being a candidate frequency band for) that user communication device. These signal measurement reports are then used by the base stations in their decision to allocate certain parts of their bandwidth to the served user communication devices and also to hand over user communication devices to other base stations (or other frequency bands/other radio access technologies (RATs)) when the signal quality does not meet the established criteria. The handing over of a user communication device might be necessary, for example, when the user communication device has moved away from the given base station, and also when an interference problem has arisen.
The 3GPP TR 36.932 (v.12.1.0) specification defines so-called small cell enhancement scenarios. ‘Small cells’ in this context refer to the coverage areas of low-power nodes (for example Pico eNBs or Femto eNBs) that are being considered for LTE in order to support mobile traffic explosion, especially for indoor and outdoor hotspot deployments. A low-power node generally refers to a node that is operating a cell (‘small cell’) with a typical transmit power which is lower than typical transmit powers used in cells of macro nodes and base stations (‘macro cells’).
Some of the small cell enhancement scenarios are based on a split control-plane/user-plane architecture (referred to as ‘C/U Split’), in which the user communication device is configured to maintain its control plane connection with the communication network via a macro cell (operating as a primary cell Tcell′ or primary cell group “PCG”) and at the same time maintain its user plane connection via one or more ‘small cells’ (operating as secondary cell ‘Scell’ or secondary cell group “SCG”) and thereby reducing the load in the macro cell. Effectively, in this case the user communication device is using two separate radio connections via two separate nodes (i.e. a macro base station and a low-power node), one for sending/receiving user data, and another one for controlling the user communication device's operations, such as mobility management, security control, authentication, setting up of communication bearers, etc. In this case, the node handling the control-plane (e.g. a macro base station) is referred to as a master base station (MeNB) whilst the node handling the user-plane (e.g. a pico base station) is referred to as the secondary base station (SeNB). Of course not all user plane data may be transmitted through the SeNB; some user plane data may also be transmitted via the MeNB.
Current user communication devices typically support multiple radio technologies, not only LTE. The user communication devices might include, for example, transceivers and/or receivers operating in the Industrial, Scientific and Medical (ISM) radio bands, such as Bluetooth or Wi-Fi transceivers. Furthermore, user communication devices might also include positioning functionality and associated circuitry, for example Global Navigation Satellite System (GNSS) transceivers and/or receivers. Both ISM and GNSS (hereafter commonly referred to as non-LTE) radio technologies use frequency bands close to or partially overlapping with the LTE frequency bands. Some of these non-LTE frequency bands are licensed for a particular use (e.g. Global Positioning Systems (GPS) bands) or might be unlicensed bands and can be used by a number of radio technologies (such as Bluetooth and Wi-Fi standards using the same range of ISM frequency bands). The manner in which these non-LTE frequency bands are used are, therefore, not covered by the LTE standards and are not controlled by the LTE base stations.
However, transmissions in the non-LTE frequency bands might, nevertheless, still cause undesired interference to (or suffer undesired interference resulting from) transmissions in the LTE bands, particularly in the overlapping or neighbouring frequency bands. When interference arises as a result of communication occurring concurrently in the same user communication device (for example, concurrent use of LTE and non-LTE radio technologies) the interference is sometimes referred to as ‘in-device coexistence (IDC) interference’ which causes an ‘in-device coexistence (IDC) situation’. Such an IDC situation can be addressed by the user communication device (possibly with assistance by the serving base station), which is referred to as an IDC solution.
In C/U Split scenarios, the serving base stations (e.g. the MeNB and the SeNB) can exchange (over the X2 interface provided between them) information relating to the configuration of the user communication devices they are serving using appropriately formatted RRC containers (inter node messages). In other words, the base stations can include RRC messages within the X2 messages sent between the base stations. The RRC containers may be used, in particular, for procedures relating to: IDC situations; counter check; (UE specific) information request/response; broadcast/multicast services (e.g. for providing an indication of interest in a service); measurement configuration and reporting and/or the like.